"It's a decorticated stem from an arborescent lycophytes (club moss) almost certainly Lepidodendron. As the plant grew the "bark" with the photosynthetic leaf bases was shed from from the lower part of the main stem or trunk, leaving a surface with this sort of diamond-shaped patterning. Lycophyte stems in this type of preservation are sometimes given the taxonomic name Knorria, but such names are not very meaningful botanically. "
from Head of Botany at Nat Mus Wales via geology/petrology .